Assessment of base shear coefficients for URM buildings: Influence of plan configuration, building height, and seismicity levels

For over a century, equivalent static analysis has been a straightforward method for evaluating the impact of earthquake loads. Seismic codes specify the ratio between the base shear force and the building's weight, V/W. These suggested ratios mainly depend on the seismicity of the area and the building's importance and are unaffected by the number of floors, seismic load direction, or plan configuration. This study examined the V/W ratio for 36 unreinforced masonry (URM) building models, comprising nine different plan configurations and ranging from one to four stories. To assess the effect of seismic activity, the research was conducted at two peak ground acceleration levels: 0.3 g and 0.25 g. The structures were modeled using macroelements, and their capacity was evaluated using nonlinear static analysis. The results demonstrate that the V/W ratio is highly sensitive to building height and plan geometry, decreasing significantly as the number of stories increases. The ratio peaks at 0.51 for a one-story, regular (Class B) building and drops to as low as 0.08 for a four-story, irregular (Class D) building. Furthermore, slender rectangular plan configurations (Class A) were found to be unstable beyond one story. A comparison with four national seismic codes reveals that these codes do not account for such variations. Some codes suggest V/W ratios as high as 0.75-0.82, potentially overestimating the seismic capacity of multi-story or irregular URM buildings by a significant margin. This study provides more realistic, configuration-dependent V/W ratios, which are essential for safer preliminary design and vulnerability assessment.